The present invention relates to an aqueous dispersion based on a silicone resin which is crosslinkable into elastomeric state by eliminating water therefrom.
U.S. Pat. No. 2,891,920 describes a process for the emulsion polymerization of polydiorganosiloxane employing an acidic or basic catalyst in the presence of anionic, cationic or nonionic surface-active agents. This patent relates that the emulsions obtained are stable in storage and, after addition of fillers, can be used to produce paints forming a continuous coating by elimination of water.
U.S. Pat. No. 3,294,725 describes, in particular, the use of dodecylbenzenesulfonic acid for polymerizing polydiorganosiloxanes in emulsion. This patent notes that, in order to obtain stable emulsions, it is desirable to control the pH of these emulsions at a value of approximately 7. The '725 patent discloses that an elastomeric coating can be obtained from these neutralized emulsions, to which colloidal silica and a polyalkoxysilane have been added.
The disclosure of U.S. Pat. No. 3,360,491 is similar to that of U.S. Pat. No. 3,294,725, except that dodecylbenzenesulfonic acid is replaced by lauryl hydrogen sulfate.
U.S. Pat. No. 3,355,406 describes, inter alia, a silicone latex comprising:
(i) a colloidal aqueous suspension of an .alpha.,.omega.-(dihydroxy)polydiorganosiloxane; PA1 (ii) a filler necessarily comprising a silsesquioxane in the form of a colloidal suspension; and PA1 (iii) a catalyst selected from among the inorganic acids, strong bases, dialkyltin diacylates and organic or inorganic peroxides. PA1 (i) an anionically stabilized emulsion of an .alpha.,.omega.-(dihydroxy)polydiorganosiloxane polymer; PA1 (ii) a siliceous filler; PA1 (iii) a tin salt; and PA1 (iv) if desired, a nonreinforcing filler. PA1 (a) they are not stable in storage when packaged in a single package (single-component composition); PA1 (b) they require an excessively long time to cure into an elastomer; PA1 (c) they are not thixotropic; and PA1 (d) they adhere only very weakly to the usual substrates. PA1 (A) 100 parts by weight of an emulsion of the oil-in-water type of an .alpha.,.omega.-(dihydroxy)polydiorganosiloxane, stabilized with at least one surface-active agent selected from among the anionic and nonionic surface-active agents and mixtures thereof; PA1 (B) 1 to 100 parts by weight of an hydroxylated silicone resin containing, per molecule, at least two different recurring structural units selected from among those of the formulae: R.sub.3 SiO.sub.0.5, R.sub.2 SiO, RSiO.sub.1.5 and SiO.sub.2, the radicals R, which are identical or different, being vinyl, phenyl or 3,3,3-trifluoropropyl radicals, or linear or branched chain alkyl radicals containing from 1 to 6 carbon atoms inclusive, said resin having a hydroxyl group weight content of from 0.1% to 10%; PA1 (C) 0 to 250 parts by weight of a nonsiliceous inorganic filler; PA1 (D) 0.01 to 3 parts by weight of a metal curing catalyst compound; and said dispersion having a solids content of at least 40%.
U.S. Pat. No. 3,697,469 describes a particular process for emulsion polymerization of polydiorganosiloxanes and indicates the possibility of adding colloidal silica and a tin salt to this emulsion in order to obtain an elastomeric coating by evaporation of water.
French Patent FR-A-2,110,358 describes a silicone emulsion having a pH of from 6.5 to 9, crosslinking into an electrically conductive elastomer after evaporation of water by incorporation of carbon black. The emulsion, additionally comprising a tin salt and a polyalkoxysilane, is not stable in storage and must be stored in two separate packages (two-component emulsion).
U.S. Pat. Nos. 4,221,688 and 4,244,849 and French Patent FR-A-2,463,163 describe silicone emulsions which are stable in storage and comprise:
The siliceous filler may be a colloidal silica (U.S. Pat. No. 4,221,688), sodium silicate (U.S. Pat. No. 4,244,849), or an amorphous silica in powder form (FR-A-2,463,163).
When compared with the known aqueous emulsions of the prior art, these three patents suggest, on the one hand, that in order to obtain a single-component emulsion which is stable in storage, the emulsion must be maintained at an alkaline pH higher than 8.5 or 9, preferably higher than 10 and, on the other hand, a tin salt must be incorporated in the emulsion to shorten to a few days the emulsion maturing stage required to obtain an emulsion which is capable of crosslinking.
Known filled silicone emulsions, stored at an acidic or neutral pH, exhibit at least one of the following disadvantages:
Aqueous emulsions or dispersions based on a silicone resin and capable of crosslinking into an elastomer by elimination of water are already known to this art.
Thus, U.S. Pat. No. 3,355,406, referred to above, describes a silicone latex consisting of an .alpha.,.omega.-(dihydroxy)polydiorganosiloxane, prepared preferably by emulsion polymerization and of a silsesquioxane resin consisting of RSiO.sub.1.5 units (hydrocarbon R residue). The latex may additionally comprise a metal curing catalyst and an alkyltrialkoxysilane.
In U.S. Pat. No. 4,554,187, the silicone resin associated with the .alpha.,.omega.-(dihydroxy)polydiorganosiloxane is a reactive resin of low molecular weight, containing alkoxy or acyloxy groups.
In published Patent Application EP-A-266,729, the silicone resin associated with the .alpha.,.omega.-(dihydroxy)polydiorganosiloxane and with the curing catalyst is a siliconate.
EP 304,719, published on Mar. 1, 1989, discloses the use, instead of the siliconate, of a silicone resin of high molecular weight, insoluble in toluene and present in the form of an aqueous emulsion in which the particle (micelle) size is greater than 200 nonometers.
A silicone resin comprising up to 10% by weight of hydroxyl group may be associated with this siliconate.